The invention relates to a radiation source for extreme ultraviolet electromagnetic radiation. More particularly, the invention relates to the use of the radiation source in a lithographic projection apparatus for imaging of a mask pattern in a mask onto a substrate.
A lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
Up to very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine. The apparatus may also have more than one mask table and may comprise components which are operated in vacuum, and are correspondingly vacuum-compatible.
Lithographic apparatus may employ various types of projection radiation, such as ultraviolet light (UV), extreme UV, X-rays, ion beams or electron beams, for example. Depending on the type of radiation used and the particular design requirements of the apparatus, the projection system may be refractive, reflective or catadioptric, for example, and may comprise vitreous components, grazing-incidence mirrors, selective multi-layer coatings, magnetic and/or electrostatic field lenses, etc; for simplicity, such components may be loosely referred to in this text, either singly or collectively, as a xe2x80x9clensxe2x80x9d.
To fulfill the demand for constantly decreasing dimensions (line widths) in the manufacture of ICs, a radiation source yielding radiation with smaller wavelengths is required. In a lithographic apparatus, the size of the features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices and hence higher operating speeds, it is desirable to be able to image smaller features. Whilst most current lithographic apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use higher frequency (energy) radiation such as extreme ultraviolet (EUV) radiation. At the moment radiation sources for EUV radiation are investigated, for instance, generating electromagnetic radiation having a wavelength in the order of 10 nm. One of the sources under investigation comprises a primary jet nozzle for ejecting a flow of a primary gas or liquid. The primary gas or liquid is then brought into an excited energy state by means of, for instance, irradiation by a laser or generation of an electrical discharge. Other means for generating the excited energy state may, however, also be employed. A laser used should have a suitable laser frequency and a suitable intensity. An electrical discharge may be generated in the primary gas or liquid by a strong electrical field between electrodes connected to a source of high electrical potential. In general, the excited energy state will be a plasma in which electrons are detached from atoms of the primary gas or liquid. Recombination of electrons and atoms will occur under the emittance of electromagnetic radiation. The electromagnetic radiation will comprise a major contribution in the EUV range of the radiation spectrum when a suitable primary gas or liquid is selected at appropriate laser or electrical excitation conditions. Having a reliable EUV radiation source yielding a high brightness is very important in lithographic projection apparatus.
Below, reference will mainly be made to a primary gas and a primary gas jet nozzle. It is to be understood that the radiation source may also comprise a primary liquid jet nozzle for a primary liquid.
An EUV radiation source only comprising a primary gas jet nozzle has a strongly diverging flow of the primary gas from the nozzle outlet, implying that the density of the primary gas strongly decreases with increasing distance to the nozzle outlet. To reach a sufficiently high EUV radiation brightness, the plasma is to be created in a rather confined high density region of the flow of gas, i.e. very close to the nozzle outlet. A major drawback of creating a plasma close to the nozzle outlet is that the plasma will interact with the nozzle, causing debris production from the nozzle. The debris production from the nozzle causes, besides a shortened nozzle lifetime, a shortened lifetime of, for instance, a condenser system for the radiation source by a deposition of debris on its constituting optical elements such as lenses and mirrors. Especially for an EUV radiation source the optical elements are very sensitive for a deposition of debris.
Another drawback of the known primary gas jet nozzle source is that plasma particulates may escape from the plasma region. These escaped plasma particulates may thermalize and neutralize at, for instance, the optical elements of the condenser system of the radiation source, which even further decreases the lifetime of the optical elements and the condenser system.
An object of the present invention is to provide an improved EUV radiation source which avoids and alleviates the drawbacks of the radiation source referred to.
According to the present invention a radiation source for extreme ultraviolet electromagnetic radiation includes:
a primary jet nozzle constructed and arranged to eject a primary gas or liquid in a first direction;
a supply for the primary gas or liquid which is to be brought into an excited energy state when ejected from the primary nozzle and is to emit ultraviolet electromagnetic radiation when falling back to a lower energy state;
an exciting mechanism for bringing the primary gas or liquid into the excited energy state;
a secondary jet nozzle constructed and arranged to eject a secondary gas or liquid in the first direction and positioned aside the primary jet nozzle; and
a supply for the secondary gas or liquid.
By having a secondary jet nozzle aside the primary jet nozzle the degree of divergence of the primary gas may be decreased by the outflow of the secondary gas from the secondary jet nozzle. Since a sufficient density in the primary gas will then be present at a larger distance from the jet nozzle outlet, the plasma can thus be created at the larger distance from the nozzle outlet. This prevents the production of debris and its associated problems mentioned above. Further, the radiation source may be positioned such that the outflow of secondary gas will function as a shield in between optical elements and the plasma created. Such a shield will largely prevent the escape of plasma particulates towards said optical elements. The particulates do not pass the screening secondary gas and are prevented from reaching any optical elements where they may cause damaging effects by thermalization and neutralization. Also, a radiation source having a reduced re-absorption of emitted radiation and an increased brightness is obtained, since the primary jet is prevented from expanding into the direction of the optical elements, such as multi-layer and grazing incident optics.
In a preferred embodiment the secondary jet nozzle encloses the primary jet nozzle. Such a configuration will yield an even better control over the divergence of the primary gas, and an outflow of the primary gas, which is parallel or even convergent over a certain distance from the nozzle outlet can be obtained. The secondary gas enclosing the primary gas and a plasma created therein also further prevents the escape of plasma particulates from the plasma. In an optimal configuration the primary and secondary jet nozzles are co-axial.
In an efficient EUV radiation source the primary gas may comprise at least one gas selected from the group comprising krypton and xenon, or the primary liquid may comprise a liquid selected from the group comprising water droplets and cryogenic liquids such as, for instance, liquid xenon. The secondary gas may comprise at least one gas selected from the group comprising helium, neon, argon, krypton, methane, silane and hydrogen. Hydrogen is the preferred secondary gas since it has superior absorption characteristics with respect to EUV radiation. It may thus be used in a large flow rate (high local density in the outflow), yielding a very efficient confinement of the primary gas for divergence control and screening of the plasma.
The present invention also provides a lithographic projection apparatus for imaging of a mask pattern in a mask onto a substrate and includes:
a radiation system for supplying a projection beam of radiation and comprising a radiation source;
a mask table provided with a mask holder for holding a mask;
a substrate table provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate, wherein in that
the radiation system comprises the radiation source described above.
According to yet a further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus which includes:
a radiation system for supplying a projection beam of radiation and comprising a radiation source;
a mask table provided with a mask holder for holding a mask;
a substrate table provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate, The method includes:
providing a mask bearing a pattern to the mask table;
providing a substrate which is at least partially covered by a layer of radiation-sensitive material to the substrate table; and
using the projection beam of irradiation to project an image of at least a portion of the mask pattern onto a target portion on the substrate, characterized in that
a radiation source as described above is used in the radiation system for supplying the projection beam of radiation.
In a manufacturing process using a lithographic projection, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference has been made hereabove to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.